etna: use gallium utility functions where possible

- util_format_get_blockwidth
- util_format_get_blockheight
- util_format_is_compressed
- util_format_is_depth_or_stencil
Added missing header, fixed CXXFLAGS c99 flag, made GCCPREFIX optional

some toolchains set the CC, CXX etc. variables directly. GCCPREFIX
is unnecessary then (and may even be counter-productive).

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>